The invention relates generally to the field of laminated structures, and more particularly to methods for making honeycomb sandwich structures and associated products with decreased core crush values and/or reduced void content. In addition, the invention relates to the starting materials utilized to assemble such honeycomb sandwich structures.
Co-cured honeycomb sandwich structures comprising a honeycomb core and at least one prepreg ply (i.e., a fabric impregnated with a resin system) disposed on each surface of the honeycomb core are used throughout the aerospace industry in order to provide high mechanical strength at low densities.
A major problem of honeycomb sandwich structures is the tendency of the honeycomb core to crush during the autoclave process in manufacture. This problem is commonly referred to as “core crush.” Core crush during the production of structures (e.g., airplane structures) renders the structure useless and increases production costs due to direct labor, delays and material expenses.
Core crush is known to occur due to differential movement during the autoclave process between the prepreg plies that comprise the honeycomb sandwich structure. This differential movement was believed by the industry to possibly occur late in the autoclave cycle when the resin system's viscosity is at a minimum. Thus, known methods utilized to reduce core crush during the autoclave process have focused on preventing the differential movement by either mechanical/physical means (i.e., using tie downs to keep the prepreg plies from differentially moving) or by chemical means focusing on the resin system (i.e., using a fast reacting resin system to permit increase of the viscosity of the resin system), or on other parameters of the autoclave process (e.g., resin system utilized, such as vacuum levels used for staging and lay-up or in-situ and post processing internal pressure). See, generally, D. J. Renn, T. Tulleau, J. C. Seferis, R. N. Curran and K. J. Ahn, “Composite Honeycomb Core Crush in Relation to Internal Pressure Measurement,” Journal of Advanced Materials, October 1995, pp.31-40 (“The resin system was shown to be the most important parameter in determining core crush”). However, known mechanical/physical means of reducing core crush may increase production costs due to increased labor costs. Moreover, known chemical means of reducing core crush focusing on the resin system or other parameters of the autoclave process have sometimes failed to provide satisfactory reduction of core crush in known honeycomb sandwich structures.
An additional problem associated with honeycomb sandwich structures made by conventional methods is their tendency, in some cases, to break down over time due to the presence of a high content of voids and/or delaminations within and between the prepreg plies of the honeycomb sandwich structure. This problem is commonly referred to as “high void content.” High void content in the prepreg plies may facilitate ingression and accumulation of moisture in the voids of the prepreg plies. When subjected to elevated temperatures (e.g., autoclave conditions), this moisture increases the pressure within the voids in the prepreg plies and expands the size of the existing voids in the resulting cured structure. Further, high void content in the cured structure provides a pathway for moisture to ingress and accumulate in the core of the structure, thereby adding weight to the structure. High void content thus tends to shorten the life of the structure and/or increase undesired properties (e.g. weight) of the structure, and increases production costs due to direct labor, delays and material expenses.
A known cause of high void content is insufficient consolidation of the components of the honeycomb sandwich structure during the autoclave process. Consolidation is known to optimally occur at high pressure (i.e., about 100 PSI) during the high temperature autoclave cycle. Consolidation of the components of a known honeycomb sandwich structure generally occur at relatively low pressures (i.e., less than about 45 PSI) because the higher pressures (i.e., greater than about 45 PSI and up to about 85 PSI) that would enhance consolidation would inadvertently cause core crush in known honeycomb sandwich structures. Thus, known methods utilized to reduce void content have generally focused on resin modifications and prepreg processing techniques to reduce moisture content and entrapped air within the prepreg. These known methods may increase production costs of honeycomb sandwich structures due to the need to process each honeycomb sandwich structure through at least two autoclave cycles. Additionally, low consolidating pressure used in these known methods may fail to sufficiently advance the consolidation of the prepregs plies with the honeycomb core.
As discussed above, known prepreg plies may have their differential movement constrained to reduce core crush in honeycomb sandwich structures produced therefrom. Known methods of constraining this differential movement have focused on mechanical/physical constraining means (i.e., using tie downs) or chemical constraining means focusing on certain parameters of the autoclave process (e.g., resin system utilized, vacuum levels used for staging and lay-up, in-situ and post processing internal pressure), as discussed above. However, as discussed above, these mechanical and chemical constraining means may increase production costs due to increased labor costs and/or may fail to at all times provide satisfactory reduction of core crush in known honeycomb sandwich structures.
Known fabric components of prepreg plies generally consist of fibers which have been sized and/or finished. Sizing of the fabric facilitates weaving of the fibers into a fabric. Finishing of the fabric enhances certain known properties of the fabric (e.g., moisture resistance) and certain mechanical properties of the prepreg ply formed from the finished fabric (e.g., tensile strength, compression strength, and adhesive characteristics to honeycomb core in honeycomb sandwich structure).
Properties generally associated with known fabric components of the prepreg ply are as follows.
Commercially available carbon-fiber based fabrics are generally sized but unfinished, with sizing concentrations of 0.5% to 1.5%+/−0.1% (by weight) depending on the type of weave employed and/or the type of end use contemplated and/or the type of sizing utilized. By contrast, commercially available glass-fiber based fabrics are sized and then finished. However, the starch-based sizing is substantially removed by baking after weaving of the fabric and before application of the finish. These glass-fiber based fabrics may have finish concentrations of 0.08% to 0.21%+/−0.018% (by weight) depending on the type of weave employed and/or the type of end use contemplated and/or the type of finish utilized. For example, commercially available glass-fiber based fabrics made utilizing an 8-harness satin weave and proprietary finishes commercially available from Clark-Schwebel™ (Anderson, S.C.) (i.e., CS 724) or Burlington Glass Fabrics™ (Alta Vista, Va.) (i.e., BGF 644, BGF 508, BGF 508A) are believed to have a finish concentration of 0.10%+/−0.02%.
Known glass-fiber based fabric components (with finish) based on a fabric having an 8-harness satin weave and a fiber areal weight of 293+/−10 g/m2 generally have an ASTM stiffness value of less than 3.0 pound foot (lb ft). An exception to this general rule is a glass-fiber based fabric finished with F-69 (Hexcel™ Corporation, Casa Grande, Ariz.), which Applicants have measured to have an ASTM stiffness value of about 9.25 lb ft, based on tests performed on a sample of an 8-harness weave glass-fiber based F-69 finished fabric having a fiber areal weight of 293+/−10 g/m2. Known carbon-fiber based fabric components (with sizing) based on a fabric having a plain weave and a fiber areal weight of 193+/−7 g/m2 generally have an ASTM stiffness value of not greater than 3.3 pound foot (lb ft).
Processing associated with known fabric components of the prepreg ply is generally as follows. Known glass-fiber based fabric components are generally finished by application of the finish, followed by heat treatment at a temperature in the range of 300° F. to 350° F.
Accordingly, there is a need for new and better honeycomb sandwich structures exhibiting reduced core crush. Additionally, there is a need for new and better honeycomb sandwich structures exhibiting reduced void content. Further, there is a need for new and better prepreg plies having constrained differential movement (e.g., during manufacture). In addition, there is a need for new and better starting materials for the honeycomb sandwich structures exhibiting reduced core crush, the honeycomb sandwich structures exhibiting reduced void content, and the prepreg plies whose differential movement is constrained.